"""
CRC16 implementation for Yubico OTP.
"""
from six import iterbytes


def crc16(data):
    """
    Generate the crc-16 value for a byte string.

    >>> from binascii import unhexlify
    >>> c = crc16(unhexlify(b'8792ebfe26cc130030c20011c89f'))
    >>> hex(~c & 0xffff)
    '0xc823'
    >>> v = crc16(unhexlify(b'8792ebfe26cc130030c20011c89f23c8'))
    >>> hex(v)
    '0xf0b8'
    """
    crc = 0xffff

    for byte in iterbytes(data):
        crc ^= byte

        for i in range(8):
            lsb = crc & 1
            crc >>= 1
            if lsb == 1:
                crc ^= 0x8408

    return crc


def verify_crc16(data):
    """
    Return true if this given byte string has a valid crc-16 residual.

    >>> from binascii import unhexlify
    >>> verify_crc16(unhexlify(b'8792ebfe26cc130030c20011c89f23c8'))
    True
    >>> verify_crc16(unhexlify(b'0792ebfe26cc130030c20011c89f23c8'))
    False
    """
    return crc16(data) == 0xf0b8